In the United States, the Federal Communications Commission (FCC) allows a restricted unlicensed use of ultra-wide bandwidth (UWB) signals for wireless communication systems, “First Report and Order,” Feb. 14, 2002. The UWB signals must be in the frequency range from 3.1 to 10.6 GHz, and have a minimum bandwidth of 500 MHz. The FCC order also limits the power spectral density and peak emissions power of UWB signals to less than −43.1 dBm/MHz.
One modulation method for UWB uses extremely short time pulses, e.g., 1/1,000,000,000 of a second or less, to generate signals with bandwidths greater than 500 MHz, which corresponds to a wavelength of about 300 mm. Wireless systems that use short pulses are commonly referred to as impulse radio (IR) systems.
As shown in FIG. 1A, four different modulation techniques are commonly used for IR systems, pulse position modulation (PPM) 11, pulse amplitude modulation (PAM) 12, on-off keying (OOK) 13, and bi-phase shift keying (BPSK) 14.
As an advantage, UWB systems achieve high data rates, and are resistant to multi-path impairments. This is due to large processing gains. Additionally, IR systems enable low cost, low duty cycle, low power transceivers that do not require local oscillators for heterodyning. Because UWB transceivers are primarily implemented in the digital domain, the UWB transceivers can be integrated in a semiconductor chip. In UWB systems, multiple transceivers concurrently share the same spectrum without interference. UWB systems are ideal for short range, high-speed networks in homes, businesses, and educational institutions. Sensor networks can also use UWB transceivers.
A time-hopping (TH) IR is described by M. Win and R. A. Scholtz, “Ultra-Wide Band Width Time-Hopping Spread-Spectrum Impulse Radio for Wireless Multiple-Access Communications,” in IEEE Trans. On Communications, Vol. 48, No. 4 Apr. 2000, pp. 679-691. In that TH-IR system, each bit or symbol is represented by Nf pulses, where Nf is a positive integer. The time to transmit a bit is Ts. This is called the symbol duration. The time Ts is further partitioned into frames Tf, and the frames are partitioned into chips Tc, corresponding typically to a pulse duration. If Nc represents the number of chips in a frame and Nf represents the number of frames in a symbol, then Ts, Tf, and Tc are related byTs=NfTf=NfNcTc.  (1)
FIG. 1B shows the relationship between the symbol time Ts 101, the frame duration Tf 102, and the chip duration Tc 103 for pulses 104 for an example prior art TH-IR waveform 110 for a ‘0’ bit, and a waveform 120 for a ‘1’ bit. Typically, the pulses are spaced pseudo-randomly among the available chips in a frame according to a “time-hopping” code to minimize multi-user interference.
As stated above, the modulation can be binary phase shift keying (BPSK). With BPSK, each bit b is represented as either a positive or negative one, i.e., b∈{−1, 1}. The transmitted signal has the form
                                          s            ⁡                          (              t              )                                =                                    ∑                              i                =                1                            ∞                        ⁢                                          ∑                                  j                  =                  1                                                  N                  f                                            ⁢                                                h                                      i                    ,                    j                                                  ⁢                                  b                                      ⌊                                          i                      /                                              N                        f                                                              ⌋                                                  ⁢                                  p                  ⁡                                      (                                          t                      -                                              jT                        f                                            -                                                                        c                          j                                                ⁢                                                  T                          c                                                                                      )                                                                                      ,                            (        2        )            where cj represents the jth value of the TH code, in a range {0, 1, . . . , Nc−1}, and b is the ith modulation symbol. Additionally, an optional sequence denoted as hi,j can be applied to each pulse in the transmitted signal to ‘shape’ the spectrum of the transmitted signal and to reduce spectral lines. The sequence, hi,j, is called a polarity scrambling sequence with values of either +1 or −1. Different amplitudes are also possible to further shape the spectrum.
FIG. 2 shows a conventional coherent TH-IR receiver 200. The receiver includes an automatic gain control (AGC) unit 210 coupled to an amplifier 220 that is connected to the receive antenna 230. The receiver also includes synchronization 240, timing control 250, channel estimation 260, MMSE equalizer 270, and decoder 280 units. Rake receiver fingers 290 input to an adder 295. Each rake receiver finger includes a pulse sequence generator, correlator and weight combiner. The rake receiver fingers reduce multipath interference.
One draw back of the above transmission scheme shown in FIG. 2 is the complexity of the receiver needed to demodulate the information. Typically, coherent receivers based on a RAKE architecture are required to receive and correctly demodulate the information. An alternative modulation format is called “transmitted reference” (TR). Transmitted reference was originally developed for narrowband carrier based communications systems, but is applicable to UWB impulse radio as well.
TR-IR systems eliminate the need for a RAKE receiver, R. Hoctor and H. Tomlinson, “Delay-Hopped Transmitted-Reference RF Communications,” IEEE Conference on Ultra Wide Band Width Systems and Technologies, 2002, pp. 265-269.” In a TR-IR system, the information is encoded as phase differences of successive pulses in the sequence. Each symbol in a TR-IR system is a sequence of time-hopped ‘doublets’ or pairs of consecutive pulses. Typically, the first pulse in the pair is referred to as a ‘reference pulse’ and the second pulse is referred to as a ‘data pulse’. The two pulses are separated by a fixed unit of time delay Td. Multiple pairs can be transmitted for one information bit. The transmitted waveform has the form
                              s          ⁡                      (            t            )                          =                              ∑                          i              =              0                        ∞                    ⁢                                    ∑                              j                =                                                      iN                    f                                    2                                                                                                  (                                          i                      +                      1                                        )                                    ⁢                                                            N                      f                                        2                                                  -                1                                      ⁢                                          h                                  i                  ,                  j                                            ⁢                                                                                     (                                                                  p                        ⁡                                                  (                                                      t                            -                                                          2                              ⁢                                                              jT                                f                                                                                      -                                                                                          c                                j                                                            ⁢                                                              T                                c                                                                                                              )                                                                    +                                                                        b                                                      ⌊                                                          2                              ⁢                                                              j                                /                                                                  N                                  f                                                                                                                      ⌋                                                                          ⁢                                                  p                          ⁡                                                      (                                                          t                              -                                                              2                                ⁢                                                                  jT                                  f                                                                                            -                                                                                                c                                  j                                                                ⁢                                                                  T                                  c                                                                                            -                                                              T                                d                                                                                      )                                                                                                                )                                    ,                                                                                        (        3        )            where Tf, Tc, hij and Nf are the same as for the TH-IR case.
FIG. 3 shows the relationship the symbol time Ts 301, the frame time Tf 308, the chip time Tc 302, and the delay time Td 307 between reference pulse 303 and reference pulse 304 for example TH-IR waveforms. Waveform 310 is for a ‘0’ bit and waveform 320 is for a ‘1’ bit.
FIG. 4 shows a conventional TR-IR receiver 400, which is significantly simpler than the TH-IR receiver. The receiver includes delay 401, multiplier 402, integrator 403, sampler 407 and decision 404 units. The receiver essentially correlates the received signal 405 with a delayed version of itself 406. Obviously, the TR-IR 400 receiver is less complex than a TH-IR receiver shown in FIG. 3. However, the reduced complexity is at the cost of requiring twice the number of pulses and the additional energy required for the reference pulses, nominally 3 dB or more.
An appropriate duration of the delay Td 307 between the reference pulse and the data pulse needs to be selected. A short delay duration can decreases multiple access interference (MAI). However, a short delay can also decrease bit error rate performance in the presence of time dispersive channels when the delay is shorter than the maximum excess delay time of the channel, as described by F. Tufvesson and A. F. Molisch, “Ultra-Wideband Communication using Hybrid Matched Filter Correlation Receivers” Proc. ICC 2004. In conventional TR-IR systems, the delay time is fixed, and cannot be changed as channel conditions change.